1. Field of the Invention
The invention relates to a control device for a vehicle continuously variable transmission (belt-type continuously variable transmission), which controls an input-side pressure and an output-side pressure to suppress a belt slip and to achieve a target speed ratio.
2. Description of Related Art
Japanese Patent Application Publication No. 2010-241239 (JP 2010-241239 A) describes a control device for a vehicle continuously variable transmission (hereinafter, continuously variable transmission). The continuously variable transmission has a pair of variable pulleys of which the effective diameter is variable. The variable pulleys are formed of an input-side variable pulley (a primary pulley, a primary sheave) and an output-side variable pulley (a secondary pulley, a secondary sheave). A transmission belt is wound between the pair of variable pulleys. An input-side pressure (primary pressure) in the primary pulley is controlled to thereby control an actual speed ratio so as to coincide with a target speed ratio. In addition, when the actual speed ratio is brought into coincidence with the target speed ratio, the primary pressure and an output-side pressure (secondary pressure) in the secondary pulley each are controlled to thereby suppress a slip of the transmission belt. The primary pressure is a value obtained by dividing a primary thrust force by a primary-side movable sheave pressure receiving area. Similarly, the secondary pressure is a value obtained by dividing a secondary thrust force by a secondary-side movable sheave pressure receiving area.
Generally, in a belt-type continuously variable transmission, a thrust force ratio (=secondary thrust force/primary thrust force) for keeping a target speed ratio is set. The primary pressure and the secondary pressure are controlled so as to achieve the thrust force ratio that gives the target speed ratio within the range in which no belt slip occurs.
In a belt-type continuously variable transmission, a fail-safe valve may be provided in preparation for a failure of a device (hydraulic circuit) associated with primary pulley-side pressure control. The fail-safe valve interrupts a path for supplying the primary pressure into a primary-side cylinder in the event of occurrence of a failure and communicates a path for supplying the secondary pressure into the primary-side cylinder. There is a case where the primary pressure and the secondary pressure are set so as to be equal to each other during operation of the fail-safe valve (during fail-safe operation). In this configuration, when the thrust force ratio is set so as to keep a bottom speed ratio (a maximum speed ratio γmax, a lowest gear ratio), the primary pressure with respect to the secondary pressure may be excessive. In addition, such a fail-safe valve may be operated during another control other than during fail-safe operation. In the case where the fail-safe valve is shared in this way, when the fail-safe valve is operated through another control while a target speed ratio (for example, the bottom speed ratio) is kept, it may be difficult to achieve the target speed ratio.